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    <p><span id="tut-modules"></span></p>
    <h1 id="modules">
      <span class="section-number">6. </span>Modules<a
        href="#modules"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h1>
    <p>
      If you quit from the Python interpreter and enter it again, the
      definitions you have made (functions and variables) are lost. Therefore,
      if you want to write a somewhat longer program, you are better off using a
      text editor to prepare the input for the interpreter and running it with
      that file as input instead. This is known as creating a <em>script</em>.
      As your program gets longer, you may want to split it into several files
      for easier maintenance. You may also want to use a handy function that
      you’ve written in several programs without copying its definition into
      each program.
    </p>
    <p>
      To support this, Python has a way to put definitions in a file and use
      them in a script or in an interactive instance of the interpreter. Such a
      file is called a <em>module</em>; definitions from a module can be
      <em>imported</em> into other modules or into the <em>main</em> module (the
      collection of variables that you have access to in a script executed at
      the top level and in calculator mode).
    </p>
    <p>
      A module is a file containing Python definitions and statements. The file
      name is the module name with the suffix <code>.py</code> appended. Within
      a module, the module’s name (as a string) is available as the value of the
      global variable <code>__name__</code>. For instance, use your favorite
      text editor to create a file called <code>fibo.py</code> in the current
      directory with the following contents:
    </p>
    <pre><code># Fibonacci numbers module

def fib(n):    # write Fibonacci series up to n
    a, b = 0, 1
    while a &lt; n:
        print(a, end=&#39; &#39;)
        a, b = b, a+b
    print()

def fib2(n):   # return Fibonacci series up to n
    result = []
    a, b = 0, 1
    while a &lt; n:
        result.append(a)
        a, b = b, a+b
    return result</code></pre>
    <p>
      Now enter the Python interpreter and import this module with the following
      command:
    </p>
    <pre><code>&gt;&gt;&gt; import fibo</code></pre>
    <p>
      This does not enter the names of the functions defined in
      <code>fibo</code> directly in the current symbol table; it only enters the
      module name <code>fibo</code> there. Using the module name you can access
      the functions:
    </p>
    <pre><code>&gt;&gt;&gt; fibo.fib(1000)
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
&gt;&gt;&gt; fibo.fib2(100)
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
&gt;&gt;&gt; fibo.__name__
&#39;fibo&#39;</code></pre>
    <p>
      If you intend to use a function often you can assign it to a local name:
    </p>
    <pre><code>&gt;&gt;&gt; fib = fibo.fib
&gt;&gt;&gt; fib(500)
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377</code></pre>
    <p><span id="tut-moremodules"></span></p>
    <h2 id="more-on-modules">
      <span class="section-number">6.1. </span>More on Modules<a
        href="#more-on-modules"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h2>
    <p>
      A module can contain executable statements as well as function
      definitions. These statements are intended to initialize the module. They
      are executed only the <em>first</em> time the module name is encountered
      in an import statement.
      <a href="#id2" id="id1" class="footnote-reference brackets">1</a> (They
      are also run if the file is executed as a script.)
    </p>
    <p>
      Each module has its own private symbol table, which is used as the global
      symbol table by all functions defined in the module. Thus, the author of a
      module can use global variables in the module without worrying about
      accidental clashes with a user’s global variables. On the other hand, if
      you know what you are doing you can touch a module’s global variables with
      the same notation used to refer to its functions,
      <code>modname.itemname</code>.
    </p>
    <p>
      Modules can import other modules. It is customary but not required to
      place all
      <a
        href="https://docs.python.org/3/reference/simple_stmts.html#import"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >import</code
        ></a
      >
      statements at the beginning of a module (or script, for that matter). The
      imported module names are placed in the importing module’s global symbol
      table.
    </p>
    <p>
      There is a variant of the
      <a
        href="https://docs.python.org/3/reference/simple_stmts.html#import"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >import</code
        ></a
      >
      statement that imports names from a module directly into the importing
      module’s symbol table. For example:
    </p>
    <pre><code>&gt;&gt;&gt; from fibo import fib, fib2
&gt;&gt;&gt; fib(500)
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377</code></pre>
    <p>
      This does not introduce the module name from which the imports are taken
      in the local symbol table (so in the example, <code>fibo</code> is not
      defined).
    </p>
    <p>There is even a variant to import all names that a module defines:</p>
    <pre><code>&gt;&gt;&gt; from fibo import *
&gt;&gt;&gt; fib(500)
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377</code></pre>
    <p>
      This imports all names except those beginning with an underscore
      (<code>_</code>). In most cases Python programmers do not use this
      facility since it introduces an unknown set of names into the interpreter,
      possibly hiding some things you have already defined.
    </p>
    <p>
      Note that in general the practice of importing <code>*</code> from a
      module or package is frowned upon, since it often causes poorly readable
      code. However, it is okay to use it to save typing in interactive
      sessions.
    </p>
    <p>
      If the module name is followed by <code>as</code>, then the name following
      <code>as</code> is bound directly to the imported module.
    </p>
    <pre><code>&gt;&gt;&gt; import fibo as fib
&gt;&gt;&gt; fib.fib(500)
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377</code></pre>
    <p>
      This is effectively importing the module in the same way that
      <code>import fibo</code> will do, with the only difference of it being
      available as <code>fib</code>.
    </p>
    <p>
      It can also be used when utilising
      <a
        href="https://docs.python.org/3/reference/simple_stmts.html#from"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >from</code
        ></a
      >
      with similar effects:
    </p>
    <pre><code>&gt;&gt;&gt; from fibo import fib as fibonacci
&gt;&gt;&gt; fibonacci(500)
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377</code></pre>
    <p>Note</p>
    <p>
      For efficiency reasons, each module is only imported once per interpreter
      session. Therefore, if you change your modules, you must restart the
      interpreter – or, if it’s just one module you want to test interactively,
      use
      <a
        href="https://docs.python.org/3/library/importlib.html#importlib.reload"
        class="reference internal"
        title="importlib.reload"
        ><code class="sourceCode python"
          >importlib.<span class="bu">reload</span>()</code
        ></a
      >, e.g. <code>import importlib; importlib.reload(modulename)</code>.
    </p>
    <p><span id="tut-modulesasscripts"></span></p>
    <h3 id="executing-modules-as-scripts">
      <span class="section-number">6.1.1. </span>Executing modules as scripts<a
        href="#executing-modules-as-scripts"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h3>
    <p>When you run a Python module with</p>
    <pre><code>python fibo.py &lt;arguments&gt;</code></pre>
    <p>
      the code in the module will be executed, just as if you imported it, but
      with the <code>__name__</code> set to <code>"__main__"</code>. That means
      that by adding this code at the end of your module:
    </p>
    <pre><code>if __name__ == &quot;__main__&quot;:
    import sys
    fib(int(sys.argv[1]))</code></pre>
    <p>
      you can make the file usable as a script as well as an importable module,
      because the code that parses the command line only runs if the module is
      executed as the “main” file:
    </p>
    <pre><code>$ python fibo.py 50
0 1 1 2 3 5 8 13 21 34</code></pre>
    <p>If the module is imported, the code is not run:</p>
    <pre><code>&gt;&gt;&gt; import fibo
&gt;&gt;&gt;</code></pre>
    <p>
      This is often used either to provide a convenient user interface to a
      module, or for testing purposes (running the module as a script executes a
      test suite).
    </p>
    <p><span id="tut-searchpath"></span></p>
    <h3 id="the-module-search-path">
      <span class="section-number">6.1.2. </span>The Module Search Path<a
        href="#the-module-search-path"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h3>
    <p>
      When a module named <code>spam</code> is imported, the interpreter first
      searches for a built-in module with that name. If not found, it then
      searches for a file named <code>spam.py</code> in a list of directories
      given by the variable
      <a
        href="https://docs.python.org/3/library/sys.html#sys.path"
        class="reference internal"
        title="sys.path"
        ><code class="sourceCode python">sys.path</code></a
      >.
      <a
        href="https://docs.python.org/3/library/sys.html#sys.path"
        class="reference internal"
        title="sys.path"
        ><code class="sourceCode python">sys.path</code></a
      >
      is initialized from these locations:
    </p>
    <ul>
      <li>
        <p>
          The directory containing the input script (or the current directory
          when no file is specified).
        </p>
      </li>
      <li>
        <p>
          <span id="index-1" class="target"></span
          ><a
            href="https://docs.python.org/3/using/cmdline.html#envvar-PYTHONPATH"
            class="reference internal"
            ><code class="xref std std-envvar docutils literal notranslate"
              >PYTHONPATH</code
            ></a
          >
          (a list of directory names, with the same syntax as the shell variable
          <span id="index-2" class="target"></span><code>PATH</code>).
        </p>
      </li>
      <li><p>The installation-dependent default.</p></li>
    </ul>
    <p>Note</p>
    <p>
      On file systems which support symlinks, the directory containing the input
      script is calculated after the symlink is followed. In other words the
      directory containing the symlink is <strong>not</strong> added to the
      module search path.
    </p>
    <p>
      After initialization, Python programs can modify
      <a
        href="https://docs.python.org/3/library/sys.html#sys.path"
        class="reference internal"
        title="sys.path"
        ><code class="sourceCode python">sys.path</code></a
      >. The directory containing the script being run is placed at the
      beginning of the search path, ahead of the standard library path. This
      means that scripts in that directory will be loaded instead of modules of
      the same name in the library directory. This is an error unless the
      replacement is intended. See section
      <a href="#tut-standardmodules" class="reference internal"
        ><span class="std std-ref">Standard Modules</span></a
      >
      for more information.
    </p>
    <h3 id="compiled-python-files">
      <span class="section-number">6.1.3. </span>“Compiled” Python files<a
        href="#compiled-python-files"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h3>
    <p>
      To speed up loading modules, Python caches the compiled version of each
      module in the <code>__pycache__</code> directory under the name
      <code>module.version.pyc</code>, where the version encodes the format of
      the compiled file; it generally contains the Python version number. For
      example, in CPython release 3.3 the compiled version of spam.py would be
      cached as <code>__pycache__/spam.cpython-33.pyc</code>. This naming
      convention allows compiled modules from different releases and different
      versions of Python to coexist.
    </p>
    <p>
      Python checks the modification date of the source against the compiled
      version to see if it’s out of date and needs to be recompiled. This is a
      completely automatic process. Also, the compiled modules are
      platform-independent, so the same library can be shared among systems with
      different architectures.
    </p>
    <p>
      Python does not check the cache in two circumstances. First, it always
      recompiles and does not store the result for the module that’s loaded
      directly from the command line. Second, it does not check the cache if
      there is no source module. To support a non-source (compiled only)
      distribution, the compiled module must be in the source directory, and
      there must not be a source module.
    </p>
    <p>Some tips for experts:</p>
    <ul>
      <li>
        <p>
          You can use the
          <a
            href="https://docs.python.org/3/using/cmdline.html#cmdoption-o"
            class="reference internal"
            ><code class="xref std std-option docutils literal notranslate"
              >-O</code
            ></a
          >
          or
          <a
            href="https://docs.python.org/3/using/cmdline.html#cmdoption-oo"
            class="reference internal"
            ><code class="xref std std-option docutils literal notranslate"
              >-OO</code
            ></a
          >
          switches on the Python command to reduce the size of a compiled
          module. The <code>-O</code> switch removes assert statements, the
          <code>-OO</code> switch removes both assert statements and __doc__
          strings. Since some programs may rely on having these available, you
          should only use this option if you know what you’re doing. “Optimized”
          modules have an <code>opt-</code> tag and are usually smaller. Future
          releases may change the effects of optimization.
        </p>
      </li>
      <li>
        <p>
          A program doesn’t run any faster when it is read from a
          <code>.pyc</code> file than when it is read from a
          <code>.py</code> file; the only thing that’s faster about
          <code>.pyc</code> files is the speed with which they are loaded.
        </p>
      </li>
      <li>
        <p>
          The module
          <a
            href="https://docs.python.org/3/library/compileall.html#module-compileall"
            class="reference internal"
            title="compileall: Tools for byte-compiling all Python source files in a directory tree."
            ><code class="sourceCode python">compileall</code></a
          >
          can create .pyc files for all modules in a directory.
        </p>
      </li>
      <li>
        <p>
          There is more detail on this process, including a flow chart of the
          decisions, in <span id="index-3" class="target"></span
          ><a
            href="https://www.python.org/dev/peps/pep-3147"
            class="pep reference external"
            ><strong>PEP 3147</strong></a
          >.
        </p>
      </li>
    </ul>
    <p><span id="tut-standardmodules"></span></p>
    <h2 id="standard-modules">
      <span class="section-number">6.2. </span>Standard Modules<a
        href="#standard-modules"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h2>
    <p>
      Python comes with a library of standard modules, described in a separate
      document, the Python Library Reference (“Library Reference” hereafter).
      Some modules are built into the interpreter; these provide access to
      operations that are not part of the core of the language but are
      nevertheless built in, either for efficiency or to provide access to
      operating system primitives such as system calls. The set of such modules
      is a configuration option which also depends on the underlying platform.
      For example, the
      <a
        href="https://docs.python.org/3/library/winreg.html#module-winreg"
        class="reference internal"
        title="winreg: Routines and objects for manipulating the Windows registry. (Windows)"
        ><code class="sourceCode python">winreg</code></a
      >
      module is only provided on Windows systems. One particular module deserves
      some attention:
      <a
        href="https://docs.python.org/3/library/sys.html#module-sys"
        class="reference internal"
        title="sys: Access system-specific parameters and functions."
        ><code class="sourceCode python">sys</code></a
      >, which is built into every Python interpreter. The variables
      <code>sys.ps1</code> and <code>sys.ps2</code> define the strings used as
      primary and secondary prompts:
    </p>
    <pre><code>&gt;&gt;&gt; import sys
&gt;&gt;&gt; sys.ps1
&#39;&gt;&gt;&gt; &#39;
&gt;&gt;&gt; sys.ps2
&#39;... &#39;
&gt;&gt;&gt; sys.ps1 = &#39;C&gt; &#39;
C&gt; print(&#39;Yuck!&#39;)
Yuck!
C&gt;</code></pre>
    <p>
      These two variables are only defined if the interpreter is in interactive
      mode.
    </p>
    <p>
      The variable <code>sys.path</code> is a list of strings that determines
      the interpreter’s search path for modules. It is initialized to a default
      path taken from the environment variable
      <span id="index-5" class="target"></span
      ><a
        href="https://docs.python.org/3/using/cmdline.html#envvar-PYTHONPATH"
        class="reference internal"
        ><code class="xref std std-envvar docutils literal notranslate"
          >PYTHONPATH</code
        ></a
      >, or from a built-in default if <span id="index-6" class="target"></span
      ><a
        href="https://docs.python.org/3/using/cmdline.html#envvar-PYTHONPATH"
        class="reference internal"
        ><code class="xref std std-envvar docutils literal notranslate"
          >PYTHONPATH</code
        ></a
      >
      is not set. You can modify it using standard list operations:
    </p>
    <pre><code>&gt;&gt;&gt; import sys
&gt;&gt;&gt; sys.path.append(&#39;/ufs/guido/lib/python&#39;)</code></pre>
    <p><span id="tut-dir"></span></p>
    <h2 id="the-dir-function">
      <span class="section-number">6.3. </span>The
      <a
        href="https://docs.python.org/3/library/functions.html#dir"
        class="reference internal"
        title="dir"
        ><code class="sourceCode python"><span class="bu">dir</span>()</code></a
      >
      Function<a
        href="#the-dir-function"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h2>
    <p>
      The built-in function
      <a
        href="https://docs.python.org/3/library/functions.html#dir"
        class="reference internal"
        title="dir"
        ><code class="sourceCode python"><span class="bu">dir</span>()</code></a
      >
      is used to find out which names a module defines. It returns a sorted list
      of strings:
    </p>
    <pre><code>&gt;&gt;&gt; import fibo, sys
&gt;&gt;&gt; dir(fibo)
[&#39;__name__&#39;, &#39;fib&#39;, &#39;fib2&#39;]
&gt;&gt;&gt; dir(sys)
[&#39;__breakpointhook__&#39;, &#39;__displayhook__&#39;, &#39;__doc__&#39;, &#39;__excepthook__&#39;,
 &#39;__interactivehook__&#39;, &#39;__loader__&#39;, &#39;__name__&#39;, &#39;__package__&#39;, &#39;__spec__&#39;,
 &#39;__stderr__&#39;, &#39;__stdin__&#39;, &#39;__stdout__&#39;, &#39;__unraisablehook__&#39;,
 &#39;_clear_type_cache&#39;, &#39;_current_frames&#39;, &#39;_debugmallocstats&#39;, &#39;_framework&#39;,
 &#39;_getframe&#39;, &#39;_git&#39;, &#39;_home&#39;, &#39;_xoptions&#39;, &#39;abiflags&#39;, &#39;addaudithook&#39;,
 &#39;api_version&#39;, &#39;argv&#39;, &#39;audit&#39;, &#39;base_exec_prefix&#39;, &#39;base_prefix&#39;,
 &#39;breakpointhook&#39;, &#39;builtin_module_names&#39;, &#39;byteorder&#39;, &#39;call_tracing&#39;,
 &#39;callstats&#39;, &#39;copyright&#39;, &#39;displayhook&#39;, &#39;dont_write_bytecode&#39;, &#39;exc_info&#39;,
 &#39;excepthook&#39;, &#39;exec_prefix&#39;, &#39;executable&#39;, &#39;exit&#39;, &#39;flags&#39;, &#39;float_info&#39;,
 &#39;float_repr_style&#39;, &#39;get_asyncgen_hooks&#39;, &#39;get_coroutine_origin_tracking_depth&#39;,
 &#39;getallocatedblocks&#39;, &#39;getdefaultencoding&#39;, &#39;getdlopenflags&#39;,
 &#39;getfilesystemencodeerrors&#39;, &#39;getfilesystemencoding&#39;, &#39;getprofile&#39;,
 &#39;getrecursionlimit&#39;, &#39;getrefcount&#39;, &#39;getsizeof&#39;, &#39;getswitchinterval&#39;,
 &#39;gettrace&#39;, &#39;hash_info&#39;, &#39;hexversion&#39;, &#39;implementation&#39;, &#39;int_info&#39;,
 &#39;intern&#39;, &#39;is_finalizing&#39;, &#39;last_traceback&#39;, &#39;last_type&#39;, &#39;last_value&#39;,
 &#39;maxsize&#39;, &#39;maxunicode&#39;, &#39;meta_path&#39;, &#39;modules&#39;, &#39;path&#39;, &#39;path_hooks&#39;,
 &#39;path_importer_cache&#39;, &#39;platform&#39;, &#39;prefix&#39;, &#39;ps1&#39;, &#39;ps2&#39;, &#39;pycache_prefix&#39;,
 &#39;set_asyncgen_hooks&#39;, &#39;set_coroutine_origin_tracking_depth&#39;, &#39;setdlopenflags&#39;,
 &#39;setprofile&#39;, &#39;setrecursionlimit&#39;, &#39;setswitchinterval&#39;, &#39;settrace&#39;, &#39;stderr&#39;,
 &#39;stdin&#39;, &#39;stdout&#39;, &#39;thread_info&#39;, &#39;unraisablehook&#39;, &#39;version&#39;, &#39;version_info&#39;,
 &#39;warnoptions&#39;]</code></pre>
    <p>
      Without arguments,
      <a
        href="https://docs.python.org/3/library/functions.html#dir"
        class="reference internal"
        title="dir"
        ><code class="sourceCode python"><span class="bu">dir</span>()</code></a
      >
      lists the names you have defined currently:
    </p>
    <pre><code>&gt;&gt;&gt; a = [1, 2, 3, 4, 5]
&gt;&gt;&gt; import fibo
&gt;&gt;&gt; fib = fibo.fib
&gt;&gt;&gt; dir()
[&#39;__builtins__&#39;, &#39;__name__&#39;, &#39;a&#39;, &#39;fib&#39;, &#39;fibo&#39;, &#39;sys&#39;]</code></pre>
    <p>
      Note that it lists all types of names: variables, modules, functions, etc.
    </p>
    <p>
      <a
        href="https://docs.python.org/3/library/functions.html#dir"
        class="reference internal"
        title="dir"
        ><code class="sourceCode python"><span class="bu">dir</span>()</code></a
      >
      does not list the names of built-in functions and variables. If you want a
      list of those, they are defined in the standard module
      <a
        href="https://docs.python.org/3/library/builtins.html#module-builtins"
        class="reference internal"
        title="builtins: The module that provides the built-in namespace."
        ><code class="sourceCode python">builtins</code></a
      >:
    </p>
    <pre><code>&gt;&gt;&gt; import builtins
&gt;&gt;&gt; dir(builtins)
[&#39;ArithmeticError&#39;, &#39;AssertionError&#39;, &#39;AttributeError&#39;, &#39;BaseException&#39;,
 &#39;BlockingIOError&#39;, &#39;BrokenPipeError&#39;, &#39;BufferError&#39;, &#39;BytesWarning&#39;,
 &#39;ChildProcessError&#39;, &#39;ConnectionAbortedError&#39;, &#39;ConnectionError&#39;,
 &#39;ConnectionRefusedError&#39;, &#39;ConnectionResetError&#39;, &#39;DeprecationWarning&#39;,
 &#39;EOFError&#39;, &#39;Ellipsis&#39;, &#39;EnvironmentError&#39;, &#39;Exception&#39;, &#39;False&#39;,
 &#39;FileExistsError&#39;, &#39;FileNotFoundError&#39;, &#39;FloatingPointError&#39;,
 &#39;FutureWarning&#39;, &#39;GeneratorExit&#39;, &#39;IOError&#39;, &#39;ImportError&#39;,
 &#39;ImportWarning&#39;, &#39;IndentationError&#39;, &#39;IndexError&#39;, &#39;InterruptedError&#39;,
 &#39;IsADirectoryError&#39;, &#39;KeyError&#39;, &#39;KeyboardInterrupt&#39;, &#39;LookupError&#39;,
 &#39;MemoryError&#39;, &#39;NameError&#39;, &#39;None&#39;, &#39;NotADirectoryError&#39;, &#39;NotImplemented&#39;,
 &#39;NotImplementedError&#39;, &#39;OSError&#39;, &#39;OverflowError&#39;,
 &#39;PendingDeprecationWarning&#39;, &#39;PermissionError&#39;, &#39;ProcessLookupError&#39;,
 &#39;ReferenceError&#39;, &#39;ResourceWarning&#39;, &#39;RuntimeError&#39;, &#39;RuntimeWarning&#39;,
 &#39;StopIteration&#39;, &#39;SyntaxError&#39;, &#39;SyntaxWarning&#39;, &#39;SystemError&#39;,
 &#39;SystemExit&#39;, &#39;TabError&#39;, &#39;TimeoutError&#39;, &#39;True&#39;, &#39;TypeError&#39;,
 &#39;UnboundLocalError&#39;, &#39;UnicodeDecodeError&#39;, &#39;UnicodeEncodeError&#39;,
 &#39;UnicodeError&#39;, &#39;UnicodeTranslateError&#39;, &#39;UnicodeWarning&#39;, &#39;UserWarning&#39;,
 &#39;ValueError&#39;, &#39;Warning&#39;, &#39;ZeroDivisionError&#39;, &#39;_&#39;, &#39;__build_class__&#39;,
 &#39;__debug__&#39;, &#39;__doc__&#39;, &#39;__import__&#39;, &#39;__name__&#39;, &#39;__package__&#39;, &#39;abs&#39;,
 &#39;all&#39;, &#39;any&#39;, &#39;ascii&#39;, &#39;bin&#39;, &#39;bool&#39;, &#39;bytearray&#39;, &#39;bytes&#39;, &#39;callable&#39;,
 &#39;chr&#39;, &#39;classmethod&#39;, &#39;compile&#39;, &#39;complex&#39;, &#39;copyright&#39;, &#39;credits&#39;,
 &#39;delattr&#39;, &#39;dict&#39;, &#39;dir&#39;, &#39;divmod&#39;, &#39;enumerate&#39;, &#39;eval&#39;, &#39;exec&#39;, &#39;exit&#39;,
 &#39;filter&#39;, &#39;float&#39;, &#39;format&#39;, &#39;frozenset&#39;, &#39;getattr&#39;, &#39;globals&#39;, &#39;hasattr&#39;,
 &#39;hash&#39;, &#39;help&#39;, &#39;hex&#39;, &#39;id&#39;, &#39;input&#39;, &#39;int&#39;, &#39;isinstance&#39;, &#39;issubclass&#39;,
 &#39;iter&#39;, &#39;len&#39;, &#39;license&#39;, &#39;list&#39;, &#39;locals&#39;, &#39;map&#39;, &#39;max&#39;, &#39;memoryview&#39;,
 &#39;min&#39;, &#39;next&#39;, &#39;object&#39;, &#39;oct&#39;, &#39;open&#39;, &#39;ord&#39;, &#39;pow&#39;, &#39;print&#39;, &#39;property&#39;,
 &#39;quit&#39;, &#39;range&#39;, &#39;repr&#39;, &#39;reversed&#39;, &#39;round&#39;, &#39;set&#39;, &#39;setattr&#39;, &#39;slice&#39;,
 &#39;sorted&#39;, &#39;staticmethod&#39;, &#39;str&#39;, &#39;sum&#39;, &#39;super&#39;, &#39;tuple&#39;, &#39;type&#39;, &#39;vars&#39;,
 &#39;zip&#39;]</code></pre>
    <p><span id="tut-packages"></span></p>
    <h2 id="packages">
      <span class="section-number">6.4. </span>Packages<a
        href="#packages"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h2>
    <p>
      Packages are a way of structuring Python’s module namespace by using
      “dotted module names”. For example, the module name
      <code>A.B</code> designates a submodule named <code>B</code> in a package
      named <code>A</code>. Just like the use of modules saves the authors of
      different modules from having to worry about each other’s global variable
      names, the use of dotted module names saves the authors of multi-module
      packages like NumPy or Pillow from having to worry about each other’s
      module names.
    </p>
    <p>
      Suppose you want to design a collection of modules (a “package”) for the
      uniform handling of sound files and sound data. There are many different
      sound file formats (usually recognized by their extension, for example:
      <code>.wav</code>, <code>.aiff</code>, <code>.au</code>), so you may need
      to create and maintain a growing collection of modules for the conversion
      between the various file formats. There are also many different operations
      you might want to perform on sound data (such as mixing, adding echo,
      applying an equalizer function, creating an artificial stereo effect), so
      in addition you will be writing a never-ending stream of modules to
      perform these operations. Here’s a possible structure for your package
      (expressed in terms of a hierarchical filesystem):
    </p>
    <pre><code>sound/                          Top-level package
      __init__.py               Initialize the sound package
      formats/                  Subpackage for file format conversions
              __init__.py
              wavread.py
              wavwrite.py
              aiffread.py
              aiffwrite.py
              auread.py
              auwrite.py
              ...
      effects/                  Subpackage for sound effects
              __init__.py
              echo.py
              surround.py
              reverse.py
              ...
      filters/                  Subpackage for filters
              __init__.py
              equalizer.py
              vocoder.py
              karaoke.py
              ...</code></pre>
    <p>
      When importing the package, Python searches through the directories on
      <code>sys.path</code> looking for the package subdirectory.
    </p>
    <p>
      The <code>__init__.py</code> files are required to make Python treat
      directories containing the file as packages. This prevents directories
      with a common name, such as <code>string</code>, unintentionally hiding
      valid modules that occur later on the module search path. In the simplest
      case, <code>__init__.py</code> can just be an empty file, but it can also
      execute initialization code for the package or set the
      <code>__all__</code> variable, described later.
    </p>
    <p>
      Users of the package can import individual modules from the package, for
      example:
    </p>
    <pre><code>import sound.effects.echo</code></pre>
    <p>
      This loads the submodule <code>sound.effects.echo</code>. It must be
      referenced with its full name.
    </p>
    <pre><code>sound.effects.echo.echofilter(input, output, delay=0.7, atten=4)</code></pre>
    <p>An alternative way of importing the submodule is:</p>
    <pre><code>from sound.effects import echo</code></pre>
    <p>
      This also loads the submodule <code>echo</code>, and makes it available
      without its package prefix, so it can be used as follows:
    </p>
    <pre><code>echo.echofilter(input, output, delay=0.7, atten=4)</code></pre>
    <p>
      Yet another variation is to import the desired function or variable
      directly:
    </p>
    <pre><code>from sound.effects.echo import echofilter</code></pre>
    <p>
      Again, this loads the submodule <code>echo</code>, but this makes its
      function <code>echofilter()</code> directly available:
    </p>
    <pre><code>echofilter(input, output, delay=0.7, atten=4)</code></pre>
    <p>
      Note that when using <code>from package import item</code>, the item can
      be either a submodule (or subpackage) of the package, or some other name
      defined in the package, like a function, class or variable. The
      <code>import</code> statement first tests whether the item is defined in
      the package; if not, it assumes it is a module and attempts to load it. If
      it fails to find it, an
      <a
        href="https://docs.python.org/3/library/exceptions.html#ImportError"
        class="reference internal"
        title="ImportError"
        ><code class="sourceCode python"
          ><span class="pp">ImportError</span></code
        ></a
      >
      exception is raised.
    </p>
    <p>
      Contrarily, when using syntax like
      <code>import item.subitem.subsubitem</code>, each item except for the last
      must be a package; the last item can be a module or a package but can’t be
      a class or function or variable defined in the previous item.
    </p>
    <p><span id="tut-pkg-import-star"></span></p>
    <h3 id="importing-from-a-package">
      <span class="section-number">6.4.1. </span>Importing * From a Package<a
        href="#importing-from-a-package"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h3>
    <p>
      Now what happens when the user writes
      <code>from sound.effects import *</code>? Ideally, one would hope that
      this somehow goes out to the filesystem, finds which submodules are
      present in the package, and imports them all. This could take a long time
      and importing sub-modules might have unwanted side-effects that should
      only happen when the sub-module is explicitly imported.
    </p>
    <p>
      The only solution is for the package author to provide an explicit index
      of the package. The
      <a
        href="https://docs.python.org/3/reference/simple_stmts.html#import"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >import</code
        ></a
      >
      statement uses the following convention: if a package’s
      <code>__init__.py</code> code defines a list named <code>__all__</code>,
      it is taken to be the list of module names that should be imported when
      <code>from package import *</code> is encountered. It is up to the package
      author to keep this list up-to-date when a new version of the package is
      released. Package authors may also decide not to support it, if they don’t
      see a use for importing * from their package. For example, the file
      <code>sound/effects/__init__.py</code> could contain the following code:
    </p>
    <pre><code>__all__ = [&quot;echo&quot;, &quot;surround&quot;, &quot;reverse&quot;]</code></pre>
    <p>
      This would mean that <code>from sound.effects import *</code> would import
      the three named submodules of the <code>sound</code> package.
    </p>
    <p>
      If <code>__all__</code> is not defined, the statement
      <code>from sound.effects import *</code> does <em>not</em> import all
      submodules from the package <code>sound.effects</code> into the current
      namespace; it only ensures that the package <code>sound.effects</code> has
      been imported (possibly running any initialization code in
      <code>__init__.py</code>) and then imports whatever names are defined in
      the package. This includes any names defined (and submodules explicitly
      loaded) by <code>__init__.py</code>. It also includes any submodules of
      the package that were explicitly loaded by previous
      <a
        href="https://docs.python.org/3/reference/simple_stmts.html#import"
        class="reference internal"
        ><code class="xref std std-keyword docutils literal notranslate"
          >import</code
        ></a
      >
      statements. Consider this code:
    </p>
    <pre><code>import sound.effects.echo
import sound.effects.surround
from sound.effects import *</code></pre>
    <p>
      In this example, the <code>echo</code> and <code>surround</code> modules
      are imported in the current namespace because they are defined in the
      <code>sound.effects</code> package when the
      <code>from...import</code> statement is executed. (This also works when
      <code>__all__</code> is defined.)
    </p>
    <p>
      Although certain modules are designed to export only names that follow
      certain patterns when you use <code>import *</code>, it is still
      considered bad practice in production code.
    </p>
    <p>
      Remember, there is nothing wrong with using
      <code>from package import specific_submodule</code>! In fact, this is the
      recommended notation unless the importing module needs to use submodules
      with the same name from different packages.
    </p>
    <h3 id="intra-package-references">
      <span class="section-number">6.4.2. </span>Intra-package References<a
        href="#intra-package-references"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h3>
    <p>
      When packages are structured into subpackages (as with the
      <code>sound</code> package in the example), you can use absolute imports
      to refer to submodules of siblings packages. For example, if the module
      <code>sound.filters.vocoder</code> needs to use the
      <code>echo</code> module in the <code>sound.effects</code> package, it can
      use <code>from sound.effects import echo</code>.
    </p>
    <p>
      You can also write relative imports, with the
      <code>from module import name</code> form of import statement. These
      imports use leading dots to indicate the current and parent packages
      involved in the relative import. From the <code>surround</code> module for
      example, you might use:
    </p>
    <pre><code>from . import echo
from .. import formats
from ..filters import equalizer</code></pre>
    <p>
      Note that relative imports are based on the name of the current module.
      Since the name of the main module is always <code>"__main__"</code>,
      modules intended for use as the main module of a Python application must
      always use absolute imports.
    </p>
    <h3 id="packages-in-multiple-directories">
      <span class="section-number">6.4.3. </span>Packages in Multiple
      Directories<a
        href="#packages-in-multiple-directories"
        class="headerlink"
        title="Permalink to this headline"
        >¶</a
      >
    </h3>
    <p>
      Packages support one more special attribute,
      <a
        href="https://docs.python.org/3/reference/import.html#__path__"
        class="reference internal"
        title="__path__"
        ><code class="sourceCode python"><strong>path</strong></code></a
      >. This is initialized to be a list containing the name of the directory
      holding the package’s <code>__init__.py</code> before the code in that
      file is executed. This variable can be modified; doing so affects future
      searches for modules and subpackages contained in the package.
    </p>
    <p>
      While this feature is not often needed, it can be used to extend the set
      of modules found in a package.
    </p>
    <p>Footnotes</p>
    <p>
      <span class="brackets"><a href="#id1" class="fn-backref">1</a></span
      ><br />
      In fact function definitions are also ‘statements’ that are ‘executed’;
      the execution of a module-level function definition enters the function
      name in the module’s global symbol table.
    </p>
    <h3 id="table-of-contents">
      <a href="https://docs.python.org/3/contents.html">Table of Contents</a>
    </h3>
    <ul>
      <li>
        <a href="#" class="reference internal">6. Modules</a>
        <ul>
          <li>
            <a href="#more-on-modules" class="reference internal"
              >6.1. More on Modules</a
            >
            <ul>
              <li>
                <a
                  href="#executing-modules-as-scripts"
                  class="reference internal"
                  >6.1.1. Executing modules as scripts</a
                >
              </li>
              <li>
                <a href="#the-module-search-path" class="reference internal"
                  >6.1.2. The Module Search Path</a
                >
              </li>
              <li>
                <a href="#compiled-python-files" class="reference internal"
                  >6.1.3. “Compiled” Python files</a
                >
              </li>
            </ul>
          </li>
          <li>
            <a href="#standard-modules" class="reference internal"
              >6.2. Standard Modules</a
            >
          </li>
          <li>
            <a href="#the-dir-function" class="reference internal"
              >6.3. The
              <code class="sourceCode python"
                ><span class="bu">dir</span>()</code
              >
              Function</a
            >
          </li>
          <li>
            <a href="#packages" class="reference internal">6.4. Packages</a>
            <ul>
              <li>
                <a href="#importing-from-a-package" class="reference internal"
                  >6.4.1. Importing * From a Package</a
                >
              </li>
              <li>
                <a href="#intra-package-references" class="reference internal"
                  >6.4.2. Intra-package References</a
                >
              </li>
              <li>
                <a
                  href="#packages-in-multiple-directories"
                  class="reference internal"
                  >6.4.3. Packages in Multiple Directories</a
                >
              </li>
            </ul>
          </li>
        </ul>
      </li>
    </ul>
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